I. Field of the Invention
This invention relates generally to implantable cardiac pacing apparatus, and more particularly to a demand pacemaker whose pacing rate is determined by physiologic need as determined by changes in stroke volume and/or respiration rate and respiration depth measured using impedance plethysmography techniques.
II. Discussion of the Prior Art
As is set forth in the "Discussion of Prior Art" of the Brian D. Pederson, et al patent application Ser. No. 07/490,392, filed 3-8-90, and entitled "Adaptive Pacemaker Utilizing Respiration Related Variation in Cardiac Chamber Volume or Pressure as a Controlling Parameter" which is being filed concurrently herewith and which is assigned to applicants' assignee, there are a relatively large number of rate adaptive pacemaker designs disclosed in the patent art in which various physiologic parameters are measured and used to adapt the pacing rate of an implantable cardiac pacer to a patient's metabolic need. The invention of the aforereferenced application, discloses the design of a rate adaptive pacemaker in which signal components present in a measured impedance waveform related to intrathoracic volume and pressure variations are used in developing a control signal for the rate adaptive pacer. That is to say, volume or pressure variations within the right ventricle or right atrial chamber attributable to respiratory activity is sensed and signal processed to derive a rate controlling signal for the pacer. In that arrangement, by utilizing a low-pass filter, higher frequency components attributable to systolic events can be discriminated against in favor of the low frequency component attributable to respiratory activity. However, because there is considerable spectral overlap of the systolic event signal and the respiration signal over the range of normal physiologic conditions, it becomes difficult to separate the two utilizing a fixed cut-off linear filter. For example, in the case of intracardiac impedance measurements where respiration is the signal of interest, if a fixed 3 dB cut-off frequency (fo) of 0.25 Hz (low-pass) is employed, the fundamental of the respiration wave will pass when the patient is at rest. However, during exercise, the breathing rate fundamental frequency can approach or exceed 60 breaths-per-minute (1 Hz) and if a filter with a fixed cut-off characteristic is being employed, the respiration component, i.e., the signal of interest, will be attenuated along with the systolic component.
It is accordingly a principal object of the present invention to provide an improved signal processing circuit for processing an intracardiac or intrathoracic impedance signal capable of separating components due to respiratory activity from components due to systolic events over a wide range of physiologic conditions.
Another object of the invention is to provide a signal processing circuit for a rate adaptive pacer in which an adaptive filter is employed, allowing the cut-off frequency of the filter to be varied as a function of heart rate.